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Todd P. Silverstein*
Background
Most introductory chemistry textbooks include in their discussion of solubility and miscibility the famous rule of thumb, "like dissolves like". The converse of this rule, that nonpolar solutes are insoluble in polar solvents, is often referred to as the hydrophobic effect. This effect forms the basis for many important chemical phenomena: the cleaning action of soaps and detergents, the influence of surfactants on surface tension, the formation of biological membranes, and the stabilization of protein structure are all based in large part on the hydrophobicity of nonpolar groups. In their explanation of the hydrophobic effect, introductory chemistry textbooks often rely primarily on the concepts of enthalpy and intermolecular forces. Because the solution process is generally discussed after enthalpy, but before entropy or free energy, authors are left with little choice but to emphasize enthalpy over entropy when explaining the thermodynamic basis of "like dissolves like" and the hydrophobic effect.
For example, one text states that octane and carbon tetrachloride are miscible because the nonpolar molecules are both "held together in the liquid phase by weak dispersion forces" and thus "are attracted to one another". However, for the very reason that both molecules experience similar intermolecular forces, AH of mixing should be close to zero. These molecules are thus not "attracted" to one another in the normal enthalpic sense of the word, but rather in an entropic sense. Spontaneous mixing of the two phases is driven not by enthalpy but by entropy.
Regarding the immiscibility of octane and water, another text states that "the value of the overall enthalpy change, A.olution, is likely to be positive, reflecting an endothermic process. In large part, it is for this reason that polar and nonpolar liquids do not mix well." In the legend of an accompanying figure, this text offers the most common (incorrect) explanation as to why oil and water don't mix: "The weak intermolecular interactions between the nonpolar molecules and water cannot overcome the very strong forces between water molecules and allow them to be miscible with water." Another version of this explanation appeared in a recent overhead projector demonstration in this Journal, entitled "Why Don't Water and Oil Mix?" The authors stated the argument this way: "The water molecules are attracted to...